Steels- Bainite and Martensitic Transformation

Question 1

When does bainite form?

Answer 1

As TR decreases, pearlite becomes finer and finer. Just above the martensitic start temperature (Ms) there is a microstructure different than fine pearlite. This is bainite

Question 2

What does bainitic transformation involve?

Answer 2

Features common to both martensitic reactions and diffusion controlled nucleation and growth mechanisms

Question 3

Upper bainite

Answer 3

Forms at higher quench temperatures. Aggregate of lath-like α with carbides precipitated parallel to the needle axis of the α. The higher temperature means greater atomic mobility so there is a massive rejection of carbon from the ferrite laths meaning that Fe3C forms between laths. Generally want to avoid

Question 4

Lower bainite

Answer 4

Lower quench temperatures. Has α plates with carbides precipitated internally at an angle roughly 55° to the major axis of the α plate. Lower temperature means less atomic mobility so harder to reject carbon from the plates. Means most Fe3C forms within the laths although a small amount forms between them. Generally good.

Question 5

Describe the product of the martensitic transformation

Answer 5

Martensite. Is a metastable transformation product formed by quenching from the austenite phase. Has acicular (needle like) structure. When carbon present formed BCT or when carbon concentration low enough forms BCC. Carbon-containing martensite is hard, brittle, highly strained. Low C or C-free martensite can be ductile and soft

Question 6

Describe the martensitic transformation

Answer 6

Shear, diffusionless, displacive, military. Parent and product phases have specific crystallographic relationship. Only a change in crystal structure. Rapid (one plate takes place in 10^-7 s to grow)

Question 7

Important temperatures for martensite

Answer 7

Ms is martensite start
Mf is martensite finish
Md is martensite start with deformation
All are temperatures

Question 8

What are the two strains required to form martensite?

Answer 8

Bain strain
Lattice invariant strain

Question 9

Describe Bain strain

Answer 9

17% contraction along the c-axis and uniform expansion of 12% along a-axes. This deforms the FCC lattice to produce BCT. This is by rigid body rotation (RB). Consider two adjacent FCC cells. The BCT unit cell forms from the two shared atoms on the top face and the central atoms on the top face to form a square, same happens on bottom face, leaving atom in centre of shared face in centre of BCT unit cell

Question 10

The two microstructural observations after full martensitic transformation

Answer 10

Habit plane is not rotated or distorted
Shape change produces a surface tilt

Question 11

Describe lattice invariant strain

Answer 11

Slip or twinning so that the BCT conforms to the original position

Question 12

Deformations of lattice to produce martensite starting with a square array of austenite

Answer 12

Bain strain effectively shears the square array so it is an array of rhombuses in an overall rhombus shape. There is then slip in each layer back towards original position (two by two). Then twinning looks like it flips some layers of rhombuses (1, 2, 1, 2) so some corners align with original positions. Slip and/or twinning are for lattice invariant deformation. Not sure if both happen for same bit of martensite

Question 13

How does the carbon concentration affect the lattice parameters: a of FCC γ, a of BCT martensite, c of BCT martensite?

Answer 13

a of FCC austenite increases linearly with increasing wt% C. So does c of BCT martensite but with steeper gradient. a of BCT martensite decreases linearly with increasing wt% C but very shallow